1. Field of the Invention
The present invention relates to an improved time-base inversion type linear phase filter of IIR (Infinite Impulse Response) type having linear phase characteristics and to a nonlinear emphasizing and a nonlinear de-emphasizing circuit each containing such an improved time-base inversion type linear phase filter.
2. Description of the Prior Art
In a time-base inversion type linear phase filter shown in U.S. Pat. No. 5,299,303 (titled "Signal Processing Apparatus For Changing the Frequency Characteristics of An Input Signal") to Atsuo Ochi et al., a pair of signals having different time-base inversion phases are filtered with a pair of IIR filters arranged in parallel and then reconstructed by alternate switching to a signal which is equivalent to a signal obtained by time-base inversion using an infinite-length memory followed by filtering.
The operation and principle of the linear phase filter are described in greater detail in the above-noted U.S. Patent.
In such a linear phase filter, the input signals of the two IIR filters are required to be time-base inverted at intervals of a period which is two times longer than the duration of the filter's impulse response and thus different in the inversion phase from each other by the impulse response duration. For producing the time-base inverted signals, it is necessary to provide two time-base inverting circuits each using a memory having a storage capacity for storing signal data in each period which is two times longer than the impulse response duration. Also another time-base inverting circuit must be provided using a memory having a storage capacity of storing signal data in each impulse response duration for inverting back the signal reconstructed by switching of the outputs of the IIR filters to an original phase form, and a memory must be provided having a storage capacity for storing signal data in a period which is four times longer than the impulse response duration for compensation for the time delay caused by the time-base inversion processings.
In total, the memory size for storing data in the period which is nine times longer than the impulse response duration is needed. If the impulse response of the IIR filter ranges from 20 to 30 clock period durations, the total memory size corresponds to more than ten thousand of gates. Accordingly, it will be not easy to construct the whole circuit on an LSI chip. It is desirable for the memory size to be as small as possible.